Centre to centre Eurostar must be quicker as Warmtoast points out. An 0650 departure gets him in to Brussels at 1007 local (2hrs 17 mins). Factoring in getting from Whitehall to Northolt and getting into Brussels from the airport it doesnt matter if you are flying in a "clapped out " 146 or a brand new shiny bizjet, train is going to be just as fast, if not faster, and a lot cheaper.